Known techniques pertinent to pulp moldings having a layered structure and containing a blowing agent or a softening agent include the technique disclosed in JP-A-10-77600. This technique features use of a blowing agent or a softening agent to make a layer containing the same soft. Contemplated predominantly for use as a cushioning material, the product obtained is too weak as a packaging material, and the technique is unfit for application to thin-walled and highly heat-insulating containers. On the other hand, the technique described in JP-A-5-263400 is known for providing a foamable cellulose material by using a blowing agent. This technique is also contemplated for application as a cushioning material so that the product is too weak for use as a packaging material and unfit for application to thin-walled and highly heat-insulating containers. Further, the products obtained by these techniques have a coarse surface which, when provided with a coating, only results in uneven coating. Furthermore, the resulting pulp moldings have so poor dimensional accuracy that they are unapplicable to packaging materials involving screwing, fitting, and the like. In addition, they easily produce paper dust due to low surface layer strength.
Accordingly, a first object of the present invention is to provide a thin-walled and highly heat-insulating container and a method of producing the same.
JP-A-10-96200 is also known as a conventional technique relating to pulp molded articles having a layered structure containing a blowing agent, in which a wet pulp molded article containing a blowing agent is placed in a drying mold having a prescribed clearance, and the blowing agent is expanded to fill the clearance. The resulting molded article has improved surface smoothness. Nevertheless, because it is a cushioning material made from a pulp slurry containing a heat expandable micro-encapsulated blowing agent, the surface is not printable, has small surface strength (scratch resistance), easily produces paper dust, and is not smooth enough to be laminated with a resin film layer. Therefore, it is unfit for application to food and drink containers, particularly those in which hot water is poured, such as cups for instant noodles.
The container of Japanese Utility Model 3065471 is also known as a conventional technique relevant to paper-made heat-insulating containers. The container is composed of an inner container sheathed with an outer container having ribs which provide a gap between the inner container and the outer container to thereby achieve heat insulation.
Although the container exhibits heat insulation owing to the gap, the inner container and the outer container are not sufficiently integral with each other due to the limited joint area therebetween, which makes the whole container poor in shape retention. Particularly when it is used as a container in which hot water is poured, the paper-made inner container itself is easily deformed by the heat and the weight of hot water. For such application, the inner container must have a thick and strong wall, which, of necessity, results in an increased weight. Therefore, the container is unfit for such application.
Additionally, it has been desired for a heat-insulating container to have necessary heat insulating properties and strength in necessary portions according to its shape, etc.
Accordingly, a second object of the present invention is to provide a heat-insulating container having heat-insulating properties and strength in desired portions according to its shape, etc. and a method of producing the same.
Further, known techniques relating to a heat-insulating pulp molded container having an inner layer, an outer layer, and a heat-insulating layer between the inner layer and the outer layer include the heat-insulating container disclosed in JP-A-11-301753. The disclosed, pulp-molded heat-insulating container has a gap serving as a heat-insulating layer between pulp-molded inner and outer layers. Since the heat-insulating layer is an air gap, the container cannot assure desired mechanical strength without increasing the strength of the inner and the outer layers by increasing the basis weight, the density, etc. of the inner and the outer layers, which necessarily results in an increase of weight of the whole container.
JP-A-2000-109123 teaches a container comprising an inner material and an outer material both comprising a paper-made base material and an intermediate layer comprising a pulp-containing foamed layer interposed therebetween via an adhesive.
However, because the pulp-containing blowing agent achieves only a low expansion ratio, it should be used in a large quantity for obtaining desired heat insulating properties, which leads to a high cost. Besides, if the thickness of the base materials is reduced in an attempt to reduce the weight of the container, the adhesive would percolate in the base materials to cause color change or swelling of the base materials. Hence, it is not allowed to apply the adhesive in amounts enough to secure sufficient adhesion between the base materials and the intermediate layer. As a result, the base materials and the intermediate layer tend to separate easily.
Accordingly, a third object of the present invention is to provide a thin and lightweight heat-insulating container which has high buckling strength as well as heat insulating properties.
Known apparatus for producing pulp molded articles include the apparatus of U.S. Pat. No. 3,235,445. The apparatus is of rotary type in which molded articles are continuously manufactured through three steps in rotation of papermaking, compression of a pulp layer, and removal of the pulp layer.
In general, a papermaking step for producing pulp molded articles is carried out by use of a papermaking screen (a silk screen is used in the disclosed apparatus), and a papermaking screen is damaged easily through repeated use. Therefore, the above-described apparatus must be suspended each time the papermaking screen is repaired or renewed, which is unfavorable as an apparatus for manufacturing heat-insulating containers at reduced cost by increasing production efficiency.
Accordingly, a forth object of the present invention is to provide an apparatus for producing a heat-insulating container which is capable of providing heat-insulating containers at high production efficiency.